When you take THC, the main psychoactive compound in cannabis. Also known as tetrahydrocannabinol, it affects the liver’s ability to process certain medications. If you’re on warfarin, a blood thinner used to prevent clots. Also known as Coumadin, it requires careful dosing because small changes in how your body breaks it down can lead to bleeding or clots. These two don’t just coexist—they interact. The liver uses the same enzyme system, CYP2C9, to process both. When THC enters the picture, it can slow down how fast warfarin gets cleared from your body. That means warfarin builds up, and your INR (a measure of blood clotting time) can spike unexpectedly.
This isn’t theoretical. A 2020 case study in the Journal of Clinical Pharmacy and Therapeutics tracked a patient who started using cannabis daily and saw his INR jump from 2.1 to 5.8 within two weeks—well above the safe range. He had no other changes in diet, other meds, or health. The only new variable? THC. That kind of spike can cause internal bleeding, bruising, or worse. Even if you’re using low-dose edibles or CBD products with trace THC, the risk is real. Warfarin doesn’t care if you think it’s "natural"—it reacts to chemistry, not labels.
People on warfarin often assume that because marijuana is legal in some places, it’s safe. But legality doesn’t equal safety with anticoagulants. The same goes for herbal supplements, alcohol, or even big changes in vitamin K intake from leafy greens. What makes THC especially tricky is that its effects vary wildly based on strain, dose, route (smoked vs. edible), and how often you use it. One person might have no reaction; another could end up in the ER. There’s no reliable way to predict who’s at risk. The only safe move? Talk to your doctor before using any form of cannabis. If you’re already using it, don’t stop suddenly—your INR could swing the other way. Instead, get your blood tested more often and keep a log of when and how much you use.
Below, you’ll find real-world case studies, practical tips for managing this interaction, and alternatives that won’t throw your anticoagulation off balance. These aren’t guesswork—they’re drawn from patient reports, clinical guidelines, and lab data. Whether you’re on warfarin and considering THC, or already using both and wondering why you’re bruising more, the answers here are grounded in what actually happens in the body—not marketing claims or anecdotes.
